1. Field of the Invention
The present invention relates to wireless communication, and particularly to short-range Bluetooth wireless communication in a scatternet ring.
2. Description of the Related Art
Bluetooth transmission is a promising new technology for short-range connectivity between mobile devices, generally used as an economical replacement for point-to-(multi)point cables. Single chip, low-power, Bluetooth communication modules that fit inside mobile phones, laptops, palm computers, digital cameras, and cordless headsets allow users to interconnect these devices easily and quickly without the need for cables. FIG. 1 is a schematic diagram showing a Bluetooth piconet (FIG. 1a) and scatternet (FIG. 1b), respectively. Devices are organized in piconets, referred to as basic Bluetooth network units. Every piconet has a master and up to 7 slaves. A group of interconnected piconets are referred to as a scatternet. Piconet 1 comprises a master 10, a slave 11, a slave 12 and a slave 13. Multiple piconets with overlapping areas of coverage can form a scatternet. A scatternet 2 comprises a piconet 21 and a piconet 22. The piconet 21 comprises a master 210, a slave 211, a slave 212, a slave 213 and a bridge 23. The piconet 22 comprises a master 220, a slave 221, a slave 222, a slave 223 and a bridge 23. The bridge 23 participates in piconets 21 and 22 and connects the two piconets to form the scatternet 2.
Bluetooth channels use a frequency-hop/time-division-duplex (FH/TDD) scheme. The channel is divided into time intervals, each of 625-μs, referred to as slots, where a different hop frequency is used for each slot. The frequency hopping occurs 1600 times per second. Devices use a transmission pattern referred to as time division multiple access (TDMA), which allows the devices to transmit one certain packet in one slot, and transmit or receive another packet in the following slot. A packet can cover one slot, three slots or five slots. For a one-slot packet, the transmission frequency is determined in accordance with the corresponding Bluetooth clock of the slot. For a multiple-slot packet, the transmission frequency is determined in accordance with the Bluetooth clock of the first slot covered by the packet, and the transmission frequency remains unchanged throughout the transmission time of the packet.
A Bluetooth unit can be treated as a bridge by acting as slave in multiple piconets, or slave in one or more piconets and master in another piconet simultaneously. However, according to the Bluetooth specification, a Bluetooth unit cannot be a master in more than one piconet. Generally, the connection between a master and a slave is established on demand initiated from the master. The communication in a piconet is controlled by the corresponding master. Every Bluetooth unit in a piconet uses the defined frequency hopping sequence determined by the master of the piconet. Every Bluetooth unit has a clock and a globally unique 48 bit Bluetooth address, assigned in accordance with the IEEE 802 standard. The Bluetooth address is an open address code, which can be acquired manually or automatically. The baseband derives a frequency hopping sequence from the Bluetooth address and the Bluetooth clock. The master informs the slave of its address and clock when it establishes a link therewith. The slave derives the hop frequency of the master in accordance with the received information and adjusts its hop frequency accordingly to synchronize with the master. A slave is allowed to transmit in a given slot if the master has addressed it in the preceding slot.
Different types of link can be established between the master and its slave(s) under different situations. Two link types have been defined: a Synchronous Connection-Oriented (SCO) link and an Asynchronous Connection-Less (ACL) link. The SCO link is a symmetric, point-to-point link between a master and a specific slave. The SCO link reserves slots and can therefore be considered as a circuit-switched connection between the master and the slave. The SCO link typically supports time-bound information like voice. The master can support up to three SCO links to the same slave or to different slaves. The ACL link is a point-to-multipoint link between the master and all its slaves. In the slots not reserved for the SCO link(s), the master can establish an ACL link on a per-slot basis to any slave. Between a master and a slave only a single ACL link can exist. For most ACL packets, packet retransmission is applied to assure data integrity.
There are four operational modes for each Bluetooth unit: active, sniff, hold, and park modes.
In the active mode, the unit actively participates on the channel. The master of the piconet schedules the transmission based on traffic requirements to and from the slaves. A slave in the active mode is allowed to use regular transmission to stay synchronized to the channel. An active slave listens for packets at predetermined intervals. An active slave stays in the standby mode until the next scheduled master transmission. A slave stays synchronized with the master by the channel access code.
In the sniff mode, the duty cycle of a slave's listening activity is reduced. A slave using an ACL link listens for the master in every ACL slot. In this mode, the master starts transmission only in a specified periodic time slot.
In the hold mode, a Bluetooth unit transmits SOC packets but not ACL packets. The unoccupied slots can operate scanning, paging, and inquiring, or participate in another piconet. In this mode, a slave hibernates for a specified amount of time based on the negotiation between the master and the slave before entering the hold mode. The slave keeps its Active Member Address (AM_ADDR) in the hold mode and returns to active mode after the specified time period.
A slave device enters park mode when there is no current need for active participation in the piconet, and there is a need for synchronization. In the park mode, a slave abandons its AM_ADDR and adopts a Parked Member Address (PM_ADDR) and an Access Request Address (AR_ADDR). The parked slave listens periodically to the broadcast traffic on the piconet to remain synchronized. When not listening, the slave enters low power mode to reduce power consumption.
Using park mode also increases the theoretical number of possible slaves from 7 to 4096. As mentioned above, units are organized in piconets, each of which has one master and up to seven active slaves. A unit can be a slave in two or more piconets and master in another piconet, but cannot be a master in more than one piconet. A unit participating in more than one piconet is referred to as a bridge. Piconets are interconnected via bridge nodes to form a wider network known as a scatternet. Multiple piconets can co-exist in a common area because each uses a different hopping sequence.
The unit stays in standby mode and continues listening to remote call messages before it interconnects with another Bluetooth unit.
In the Bluetooth wireless specification, the structure of scatternet has not been established, since protocols for inter-piconet communication among units in a scatternet have not yet been defined. A plausible solution is to model a scatternet as an instance of an ad hoc network and use already known methods of routing. While this approach is certainly feasible, it is not an efficient solution. The Bluetooth layer presents a unique set of features and limitations for which known ad hoc routing protocols are not optimized.
Hence, there is a need for simple and efficient scatternet topologies with well supports of routing protocols.